The present invention relates to a storage device provided with memory units having a data compressing function.
A storage device usually has physical memory units for storing data and a controller for controlling the physical memory units. The controller makes data storage spaces (generally logical volumes) available to a computer (e.g. a host computer) connected to the storage device.
A storage device, by using plural physical memory units in a RAID (redundant array of independent (or inexpensive) disks) configuration, increases the speed of I/O processing and achieves high resistance to troubles with physical memory units.
Whereas a storage device usually has a large number of hard disk drives (HDDs) as physical memory units, today physical memory units having flash memories (FMs) as physical memory media, such as solid state drives (SSDs), in place of HDDs are attracting note. The SSD has an advantage of being capable of I/O processing far faster than HDDs.
Over the recent years, the volume of data stored into a storage device has kept on increasing. Reflecting this trend, data compression technology to reduce the volume of data to be stored and thereby to save the cost of storage devices is attracting note. As SSDs, in particular, are higher than HDDs in cost per bit, there is a keen need for reducing the volume of stored data and increasing the apparent storage capacity by using appropriate compression technology.
Japanese Unexamined Patent Application Publication No. Hei 8(1996)-249128 (Patent document 1) discloses a technique by which data received from a host computer by the controller of a storage device (hereinafter referred to as storage controller) are compressed within the storage controller and compressed data are stored into an HDD.
PCT/JP2011/003204 (Patent document 2) discloses a technique regarding a flash memory storage (hereinafter referred to as SSD) in which a controller within an SSD (hereinafter referred to as SSD controller) has a data compressing function and whose virtual logical capacity made available to an external device (storage controller) varies with the data compression rate. Namely, the smaller the data size after compression, the greater the virtual logical capacity available from the SSD to the external device. As the data size after compression depends on the pattern of the data to be compressed, it is not constant. For this reason, where the storage device is mounted with plural SSDs, the virtual logical capacity that can be made available to external devices can differ from one SSD to another.